Bremsstrahlung isochromat spectroscopy BIS

Inverse Photoemission Spectroscopy (IPES) and Bremsstrahlung Isochromat Spectroscopy (BIS)... 

Various photoemission techniques are a powerful tool for the investigation of especially large-scale features of the occupied electronic states. Numerous photoemission data are available mainly on the uranium intermetallics. These were obtained by all possible kinds of excitations such as X-ray photoemission (XPS), ultraviolet high-resolution photoemission (UPS), or synchrotron-radiation-excited photoemission with the possibility of tuning the energy of the incident radiation. The Bremsstrahlung isochromat spectroscopy (BIS) is a probe of the empty electronic states above EF. 

The comparison between theory and experiment was extended to various spectroscopies besides XPS, i.e. resonant photoemission spectroscopy. X-ray absorption spectroscopy (XAS) and, as we shall see in chapter 70 of this volume, also to bremsstrahlung isochromat spectroscopy (BIS). With one set of parameters it became possible to describe many experimental results, which in turn made it possible to extract new values of J and f. These values turned out to be quite... 

In bremsstrahlung isochromat spectroscopy (BIS) the system is exposed to an electron beam and the transiticms of the incoming electrons under the emission of photons are studied. In this case it is assumed that the initial state is of the form... 

A similar two-peak structure is seen in the inverse photoemission (bremsstrahlung isochromat spectroscopy (BIS)) spectra of cerium (Land et al. 1981) and several of its compounds (Allen et al. 1983). (In a BIS experiment, one adds an electron to the system.) The first peak is just above p ( 0.5eV) and is dominant in the more itinerant cerium compounds. The second peak is located 4-5eV above the pand is due to localized f -> P transitions. The BIS spectrum in certain ways thus mirrors what is seen in the photoemission spectrum. With this in mind, it was decided to apply the techniques of the preceding sections to determine the inverse photoemission spectrum of CeP (Norman, 1985). 

Valence band photoelectron spectroscopy results must not, of course, be viewed in isolation. In particular, bremsstrahlung isochromat spectroscopy (BIS) and X-ray photoelectron spectroscopy are complementary techniques. These and other spectroscopies are discussed elsewhere in this volume (chapters 62, 63, 70, 71 and 72) (see also Baer 1984). XPS core level results show a rich satellite structure which yields additional information, and valence band results resemble the density of states in delocalized electron systems. BIS spectra reveal the empty states which are not directly accessed by most other spectroscopies. Thus, a combination of BIS and either XPS or UPS provides spectroscopic coverage of the states on both sides of the Fermi level. 

Fig. 1. Probing the unoccupied states by bremsstrahlung isochromat spectroscopy (BIS). The number of emitted photons is proportional to the density of final states (DOS) weighted by a transition matrix element.

An average valence of v = 2.83 is derived for nearly stoichiometric TmSe from the lattice constant and v = 2.62 from the effective magnetic moment, Kobler et al. [16]. The value v = 2.62 is also deduced from the X-ray Lm absorption spectrum by Bianconi etal. [5, 17, 18], and v = 2.6 0.08 from bremsstrahlung isochromat spectroscopy (BIS) by Oh, Allen [19]. A value of v = 2.62 0.15 is obtained for the same crystal (Tm osSe, a = 5.715 A) by resonant-photoemission spectroscopy with use of synchrotron radiation in the soft X-ray region (70 to 200 eV) and constant-final-state technique (CFS), Oh et al. [41]. The valence v = 2.77 is obtained from the lattice constant and 2.46 or 2.58 from magnetic data between 77 and 300 K or 400 and 800 K, respectively, Holtzberg et al. [20]. These values are nearly confirmed for TmSe with a = 5.713 A... 

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